In the case of, for example, a rod-shaped sushi (bou-zushi), vinegared rice is shaped, and the shaped rice section is topped with a sushi material or materials such as boned and prepared sea bream, sockeye salmon or mackerel together with a thin slice of kombu kelp (shiroita kombu) cooked in a broth seasoned with vinegar, sugar and salt, and resultant is shaped into any of various shapes such as the rod-shape. The shaped sushi product is wrapped with a bamboo sheath and placed in a packing bag made of a plastic film and vacuum-packed, the vacuum-packed sushi product is frozen with a brine such as ethanol, or the shaped sushi product is frozen, and the frozen shaped sushi product is placed in a packing bag made of a plastic film and vacuum-packed, to thereby prepare a vacuum-packed frozen sushi product.
Such a packed frozen sushi product by means of brine freezing is prepared by placing a sushi product in a plastic packing bag, vacuum-packing the sushi product, and brine freezing the vacuum-packed sushi product. Accordingly, no attachment of bacteria to the sushi product or no absorption of odor from the coolant gas in the sushi product in a freezing room is caused, and therefore, the packed frozen sushi product is hygienically superior to, for example, a packed frozen sushi product prepared by vacuum-packing a sushi product posterior to freezing of the sushi product by slow freezing such as air-blast freezing, nitrogen or carbon dioxide freezing. In particular, such a packed frozen sushi product frozen by brine freezing can be stored stably, for example, over a period as long as 3 months or longer when stored at a storage temperature of −18° C., or can be stored for 6 months or longer when stored at a storage temperature of −50° C.
However, a vacuum-packed frozen sushi product which has been stored in such a manner is thawed and then served. Methods for thawing a packed frozen sushi product include natural thawing, running water thawing, steam thawing, hot water thawing and microwave thawing. In these methods, microwave thawing is suitable for thawing of a frozen sushi product or the like because of its extremely short thawing time and high thawing yield. For example, in the case of a rod-shaped sushi product as described above, the thawing thereof at room temperature requires five hours, whereas microwave thawing can thaw the rod-shaped sushi product in 2 to 5 minutes. Accordingly, microwave thawing can immediately meet the demands and thus is convenient as compared with thawing at room temperature (see Japanese Unexamined Patent Publication No. Hei 8 (1996)-214805, Japanese Unexamined Patent Publication No. Hei 8 (1996)-317764, Japanese Unexamined Patent Publication No. Hei 9 (1997)-47242, Japanese Unexamined Patent Publication No. 2001-275591 and the specification of U.S. Pat. No. 5,861,184).
However, in the case of microwave thawing, heat values generated by microwave absorption are different between the frozen sushi material section and the frozen rice section, and these are also different in quantity. Further, because a surface portion of sushi product is intensively heated as compared with a central portion of sushi product, a portion of the sushi material or materials is heated to an undesirably high temperature as compared with the rice section. The sushi material or materials are thereby brought into the so-called cooked condition, and the tastes thereof are impaired. Therefore, microwave thawing has not been practically used. In the case of, for example, a box-shaped sushi product or a rod-shaped sushi product, in general, a rice section has preferably body temperature, for example, the rice section preferably has temperatures of 15 to 25° C., more preferably 15 to 20° C. although this is dependent upon the type of the sushi product, whereas a sushi material or materials, for example, conger eel or eel is preferred to have roughly the same temperatures as the rice section, for example, temperatures of 10 to 25° C., more preferably 10 to 20° C. although this is dependent upon the type of the sushi material or materials. However, when a packed frozen sushi product is thawed by means of a microwave oven, a sushi material or materials are heated to relatively high temperatures to impair the taste of the sushi product. Accordingly, it is difficult to effect desired thawing by means of a microwave oven. Under such circumstances, with a view to solving the problem that in thawing a vacuum-packed frozen sushi product by means of microwave heating by a microwave oven or the like, a sushi material or materials of the sushi product are liable to be heated to relatively high temperatures to thereby impair taste of the sushi product, the present inventors made a proposal to prepare a so-called vacuum-packed frozen sushi product suitable for frozen storage by wrapping a sushi product topped with a sushi material or materials with a plastic wrapping film, placing the wrapped sushi product in a plastic film packing bag for frozen foods, vacuuming and hermetically seal the packing bag to vacuum-pack the sushi product, and freezing the sushi product in the hermetically packed state in whole by brine-freezing in a unified form, and in thawing the thus prepared packed frozen sushi product by means of a microwave oven, to effect thawing by heating the sushi product by a microwave oven in a relatively short time, and terminating the heating by the microwave oven so as not to raise temperatures of the sushi product under the thawing to high temperatures, and heating the sushi product with water vapor emanating from a heated rice section and prevailing in the plastic wrapping film package and the heat of the rice section while steaming, during the microwave heating and even after removal of the sushi product from the microwave oven, so as to bring a temperature of a portion where the sushi material or materials and the rice section are in contact with each other to 15° C. or higher.
For preparing such a vacuum-packed frozen sushi product, however, it is necessary to place a sushi product on a plastic wrapping film, tightly wrap the sushi product with the plastic wrapping film, place the wrapped sushi product in a plastic film packing bag for frozen foods, and vacuum and hermetically seal the packing bag. This procedure should be performed carefully, and such a packed frozen sushi product is difficult to prepare on a large scale. Accordingly, there is a problem that production of such a packed frozen sushi product to meet customers' demands involves many manual labors and much time.
It is an object of the present invention to provide a vacuum-packed sushi product which is capable of solving the problems inherent in a conventional vacuum-packed frozen sushi product wrapped with a plastic film and which is suitable for thawing by means of a microwave oven.